1. FIELD OF THE INVENTION
This invention relates to magnetic bubble domain memory systems that depend upon the generation and propagation of magnetic bubble domains in the processing of data, and more particularly to a bubble propagation path structure of gapless configuration along which magnetic bubble domains may be selectively propagated with reduced drive field requirements.
2. DESCRIPTION OF THE PRIOR ART
Magnetic bubble domain technology involves the processing of data by the storage of data bits in the form of magnetic bubbles which are moveable in a thin film of magnetic material. These magnetic bubbles are cylindrical magnetic domains whose polarization is opposite to that of the thin magnetic film or layer in which they are disposed. The magnetic film or layer is made of a material which causes the film to have magnetically easy directions essentially perpendicular to the plane of the film. The magnetic properties of this film, e.g. magnetization, anisotropy, coercivity, mobility, are such that the film may be maintained magnetically saturated with magnetization in a direction out of the plane and that small localized regions of polarization aligned opposite to the general polarization direction may be supported. Such localized regions are the magnetic bubble domains of generally cylindrical configuration referred to previously.
These magnetic bubbles can be manipulated by varying the surrounding magnetic field. For example, the magnetic bubbles may be formed in thin platelets having uniaxial anisotropy with the easy magnetic axis perpendicular to the platelet or film which may comprise such material as rare earth orthoferrites, rare earth aluminum and gallium substituted iron garnets and rare earth cobalt or iron amorphous alloys. Since the magnetic bubbles can be generated, propagated, erased, replicated, and manipulated to form data processing operations and their presence and absence detected, these bubbles may be utilized to perform many of the on-off or primary functions necessary in a binary computer operation.
The relative size of the individual magnetic bubble domains is determined by the strength of the external bias magnetic field perpendicular to the magnetic film. As this external bias magnetic field is increased in strength, the magnetic bubbles are initially caused to form as localized regions at a certain field strength. Continued increase in the strength of the external magnetic field causes the individual bubbles to shrink until they completely disappear.
Typically, the controlled movement of such magnetic bubble domains is accomplished by employing an overlay pattern of magnetizable material (e.g. permalloy) on the surface of the magnetic layer in which the magnetic bubble domains are disposed. This magnetizable overlay pattern is capable of generating magnetic poles in response to magnetic fields in the plane of the magnetic layer and the overlay material. An in-plane rotating magnetic field within the plane of the magnetizable overlay pattern is provided for the purpose of moving bubble domains along selected bubble propagation paths. The rotation of the in-plane magnetic field causes it to be re-oriented, thereby causing movement of the magnetic poles generated in the permalloy material of the magnetizable overlay pattern. Consequently, the magnetic bubble domains are also caused to move by virtue of being attracted by the pulls generated in the permalloy material in response to the changing positions of the in-plane magnetic field.
Heretofore, a common form of the magnetizable overlay pattern defining the bubble propagation paths in a magnetic bubble domain system has comprised alternating series of bar shaped and T-shaped permalloy overlay elements which are respectively spaced apart providing gaps therebetween. This type of arrangement provides a bubble propagation path in which the magnetic bubbles may be propagated in either direction along the path in response to the orientation of the in-plane rotating magnetic field. Such bubble propagation path structures have been satisfactory when employed with magnetic bubble domains having a diameter of about 5 microns which would be formed by an external bias magnetic field of approximately 200 gauss applied perpendicular to the magnetic film. In this connection, the gaps between successive magnetizable elements defining the bubble propagation path form energy barriers that hinder movement of the respective bubbles thereacross. However, the magnitude of such energy barriers in relation to the 5 micron diameter bubbles is relatively low so as to offer no appreciable resistance to the passage of the respective bubbles thereacross. Therefore, a high driving requirement from the in-plane rotating magnetic field has not been necessary to impart movement to the individual bubbles across the gaps in the propagation paths.
In order to achieve conservation in space of such magnetic bubble domain memory systems, efforts have been made to reduce the diameter of individual magnetic bubble domains so as to afford a more closely packed arrangement of such bubbles in a memory system. This enables the total number of bits in such a memory system to be substantially increased without a corresponding increase in the size of the memory system. To this end, the average bubble diameter has been reduced to the size of about 0.5 microns in some instances, as obtained through the imposition of an external bias magnetic field of increased strength to approximately 1600 gauss on the magnetic film in which the bubbles are disposed. However, the energy barriers existing in the gaps between conventional bubble propagation path structures employing spaced apart elements of a magnetizable overlay pattern become a more significant problem upon the reduction in size of the individual magnetic bubbles. In this respect, each of the energy barriers is proportional to the external fringing magnetic field from the surface poles of the bubbles. Thus, the energy barriers across the gaps in a bubble propagation path are significantly higher where magnetic bubbles of 0.5 microns in diameter are employed and must be moved across the succession of gaps existing between the magnetizable elements of the magnetizable overlay pattern defining the bubble propagation path. Consequently, relatively high drive requirements are imposed on the in-plane rotating magnetic field in order to permit the smaller-sized magnetic bubble domains to successfully traverse the gaps in the bubble propagation path along which they are intended to travel.